1. Field of the Invention
The present invention relates to a piezoelectric thin film including a piezoelectric layer and a method of manufacturing the same. The present invention further relates to an ink jet head including the piezoelectric thin film and a method of forming an image with the head, to an angular velocity sensor including the piezoelectric thin film and a method of measuring an angular velocity with the sensor, and to a piezoelectric generating element including the piezoelectric thin film and a method of generating electric power with the element.
2. Description of Related Art
Lead zirconate titanate (PZT: Pb(ZrxTi1-x)O3, 0<x<1) is a typical ferroelectric material capable of storing a large amount of electric charge. PZT is used in capacitors and thin film memories. PZT has pyroelectricity and piezoelectricity based on the ferroelectricity thereof. PZT has high piezoelectric performance. A mechanical quality factor Qm of PZT can be controlled easily by adjusting the composition or adding an element thereto. These facts allow PZT to be applied to sensors, actuators, ultrasonic motors, filter circuits, and oscillators.
PZT, however, contains a large amount of lead. In recent years, there has been a growing concern that lead leached from waste may cause serious damage to the ecosystem and the environment. Accordingly, there has been an international movement toward restricting the use of lead. For this reason, non-lead-containing ferroelectric materials (lead-free ferroelectric materials) unlike PZT have been in demand.
One example of the lead-free ferroelectric materials that are currently under development is a perovskite-type composite oxide [(Bi0.5Na0.5)1-yBay]TiO3 made of bismuth (Bi), sodium (Na), barium (Ba), and titanium (Ti). JP 4(1992)-60073 B and T. Takenaka et al., Japanese Journal of Applied Physics, Vol. 30, No. 9B, (1991), pp. 2236-2239 disclose that this ferroelectric material exhibits high piezoelectric performance of about 125 pC/N in terms of a piezoelectric constant d33, when the content of barium y(=[Ba/(Bi+Na+Ba)] is 5% to 10%. The piezoelectric performance of the ferroelectric material is, however, lower than that of PZT.
JP 2007-266346 A, H. W. Cheng et al., Applied Physics Letters, Vol. 85, (2004), pp. 2319-2321, and Z. H. Zhou et al., Applied Physics Letters, Vol. 85, (2004), pp. 804-806 disclose that a (Bi,Na,Ba)TiO3 film that is oriented in a specific direction is fabricated. It is expected that the alignment of the polarization axes in the (Bi,Na,Ba)TiO3 film with the orientation improves the ferroelectric properties of the film, such as remanent polarization and piezoelectric performance.
However, unlike a (Bi, Na, Ba)TiO3 bulk, the (Bi, Na, Ba)TiO3 thin film has a leak current. T. Takenaka et al., Japanese Journal of Applied Physics, Vol. 30, No. 9B, (1991), pp. 2236-2239 discloses a (Bi, Na, Ba)TiO3 disc with a thickness of 1 mm and a dielectric loss tan δ of about 1%. On the other hand, Z. H. Zhou et al., Applied Physics Letters, Vol. 85, (2004), pp. 804-806 discloses that a (Bi, Na)TiO3 thin film has a dielectric loss reaching 20% in a low frequency region of kilohertz or less. The ferroelectric properties of the (Bi, Na, Ba)TiO3 film, which has a large leak current, are deteriorated significantly. Thus, it is necessary to suppress the leak current of the (Bi, Na, Ba)TiO3 film.
JP 2007-019302 A discloses a piezoelectric thin film in which a NaNbO3 film is sandwiched between a substrate and a piezoelectric layer composed of a niobic acid compound ((Na, K, Li)NbO3).